1. Field of the Invention
The invention relates to a transmission projection screen comprising a transparent substrate having a front surface which is provided with mutually parallel ribs from which light issues during operation of the screen, grooves being present between the successive ribs, in which grooves light-absorbing thread-like elements are disposed.
The invention also relates to a method of manufacturing a transmission projection screen.
2. Description of the Prior Art
Such a screen is used, for example, for displaying images or alphanumeric data which are projected on the rear surface of the screen, for example, by a lens system with one or more cathode ray tubes or by a film projector. The transparent ribs situated on the front surface focus the light rays which are received from the rear side and which issue via the apexes of the ribs. The grooves between the ribs remain dark. In order to avoid reflection of ambient light from both the front side and the rear side of the screen, which would reduce the contrast of the image to be displayed, a light-absorbing material is disposed in the grooves. It is known that for this purpose a black lacquer layer can be applied or that black particles are disposed in the grooves. A disadvantage of such screens is the high degree of mechanical contact and, thus, optical contact between the ribs and the light-absorbing material, which causes light that should be reflected on the inside of the walls of the ribs to be absorbed, thereby reducing the light output of the screen.
U.S. Pat. No. 4,525,029 describes a transmission projection screen and a method of manufacturing such a screen, in which method black wires, for example, of yarn, metal or synthetic resin are introduced into the grooves. The black wires are longitudinally introduced into the grooves, during and/or after which operation the wires are under tension to make sure that they are in the grooves over their full length. The wires are fixed by means of, for example, glue. A disadvantage of such a screen is that it may be subject to warpage, for example, due to changing ambient conditions, humidity being a particularly important factor therein.
It is an object of the invention to provide a transmission projection screen which is not or hardly subject to warpage caused by changes in humidity and/or temperature. For this purpose, it is an object of the invention to provide a transmission projection screen in which the light-absorbing wires do not have to be under tension during or after their insertion. A further object of the invention is to provide a transmission projection screen having an aesthetically attractive, smooth front surface. A particular object of the invention is to provide a transmission projection screen having a flat or substantially flat front surface, the light-absorbing wires being situated deeply in the grooves over their full length so as not to catch the light issuing from the apexes of the ribs.
This object is achieved in accordance with the invention by a transmission projection screen as described in the opening paragraph, which screen is further characterized in that the light-absorbing elements are made of glass fibers.
Due to the great rigidity of glass, glass fibres can be deeply inserted into the grooves over their full length without being under tension. Thanks to the use of glass fibres an aesthetically attractive screen is obtained having straight and regular light-absorbing lines.
The surface of the glass fiber may be provided, for example, with a black layer. In a particularly suitable embodiment of the projection screen in accordance with the invention, the glass fiber consist of black glass.
The use of glass fibers has the additional advantage that the diameter of a glass fiber may be chosen within wide limits and may be adapted to the dimensions of the grooves in order to make sure that the glass fiber fits in the lower part of a groove, while having a sufficiently large diameter to substantially reduce the light reflection at the front side of the screen. By making sure that the contact area between the glass fibers and the walls of the grooves is small, it is possible to obtain only a marginal reduction in the light output of the screen.
An additional object of the invention is to provide a method of manufacturing a transmission projection screen, in which the light-absorbing elements can be readily fixed in the grooves.
This object is achieved in accordance with the invention by a method which is characterized in that it comprises the following steps:
a light-absorbing glass fiber is covered with a synthetic resin coating, PA1 synthetic resin coated glass fibers are disposed in the grooves of the screen, PA1 the screen including the glass fibers is subjected to a treatment which causes the synthetic resin coating to adhere to the walls of the grooves.
Such a treatment may consist in, for example, contacting the screen with the vapour of a solvent for the synthetic resin of the synthetic resin coating.
In a very suitable embodiment of the method in accordance with the invention, the treatment for making the synthetic resin coating adhere is a heat treatment. In this case, the synthetic resin coating consists of a glue whose adhesive action is obtained by heating and subsequent cooling, a so-called "hot-melt" adhesive. An important advantage of this embodiment of the method is that glass fibers may be used which are provided with a synthetic resin coating in a separate treatment and which at some later time, for example after storage, are used in the actual manufacture of the screen, which is usually carried out in a clean room. Consequently, chemicals such as adhesive, solvents or black powdered material for providing the grooves of the projection screen with light-absorbing material do not have to be used in the clean room.